1. Field of the Invention
The invention relates to a power converter, and in particular, to a vehicle power converter for use in a vehicle rotary motor mounted on a vehicle such as a car.
2. Description of the Related Art
Full-wave rectification using a diode applied as a rectifying element is a generally known system used in a vehicle rotary motor. Meanwhile, a recent rectifying system uses a switching element as a rectifying element to enhance efficiency, and reduces loss generated by the rectifying element.
In a vehicle power converter already suggested, for example, in Patent Document 1, a switching element is used as a rectifying element. In this power converter, ON and OFF of the switching element are controlled on the basis of the rotational position of a rotor of a vehicle rotary motor determined by rotational position detecting means.
In a system suggested, for example, in Patent Document 2, a position signal indicative of the rotational position and the angle of a rotary motor is compared with ON and OFF signals of a diode of each phase in each of higher and lower arms. This determines a time when the diode of this phase is turned on in a next cycle. A switching element of the same phase is turned on at the determined time to predict a time when the diode is turned off. Then, the switching element of the same phase is turned off at a time before the predicted time when the diode is turned off.    Patent Document 1: Japanese Patent Application Laid-Open No. 2002-218797    Patent Document 2: Japanese Patent Application Laid-Open No. 2008-228450
However, the system for determining the rotational position of a rotor of a vehicle rotary motor by using rotational position detecting means to make phase control in the ON-OFF operation of a switching element cannot respond to the need to change a time when the switching element is turned on or off in real time in response to load variations.
In order to determine the rotational position of a rotor of a vehicle rotary motor, a rotational position sensor or an angle sensor typified by a resolver should be attached to the shaft of the rotary motor, or, an RD converter for converting the output of a sensor to data of a rotational position or an angle should be prepared.
External noise may be superimposed on a position signal depending on the degree of accuracy in attaching a rotational position sensor such as a resolver, as a result of enhanced efficiency in synchronous rectification in fast-speed rotation caused by the response characteristic delay of an RD converter, or depending on the response characteristic of an RD converter in a noise region. As a result, chattering may be caused when a switching element is turned on, or, a switching element may be turned on in a period during which the switching element should be off, leading to variations in load torque of a rotary motor.
Turning a switching element on in a period during which the switching element should be off may result in the following. A current flows in a direction opposite to its original direction at the time of generation of electricity, or a short circuit is generated depending on a time when the switching element is turned on, or a period during which the switching element is on. These will cause large variations or drop in output voltage of a power source.
The following way is regarded as a simple process if attention is focused only on one phase. In this system, a position signal indicative of the rotational position and the angle of a rotary motor is compared with ON and OFF signals of a diode of each phase in each of higher and lower arms. This determines a time when the diode of this phase is turned on in a next cycle. A switching element of the same phase is turned on at the determined time to predict a time when the diode is turned off. Then, the switching element of the same phase is turned off at a time before the predicted time when the diode is turned off. However, an actual situation of this way is that a check should be made to see whether ON and OFF of a diode are detected in correct order in six phases including three phases in each of higher and lower arms.
This check requires the following determinations for each cycle: a determination as to whether the ON and OFF cycles of diodes of the six phases are substantially the same; a determination as to whether turning-on of a diode is detected precisely in U, V, and W phases in this order, in such a way that a phase difference is about 120 degrees in terms of electrical angle; a determination as to whether detection of turning-on of diodes is shifted 180 degrees in terms of electrical angle between higher and lower arms of the same phase; and a determination as to whether a period of time during which a switching element is on is substantially the same in the six phases. This check places a heavy burden on the process.